In the art of making micro-crystal cadmium sulfide voltaic cells it has been the practice to fabricate the cadmium sulfide layer of considerable thickness, say 20 microns. This has been deemed necessary to assure that pin holes, or other types of defects, do not occur in the layer which, if present, render the cell inoperative. It has heretofore been considered unfeasible to utilize extremely thin layers of cadmium sulfide because a large proportion of the cells prove defective in practice. It is one purpose of the present invention to provide a large area photo-voltaic cell capable of being incorporated in a system employing areas of photovoltaic generators covering areas of the order of square miles, to enable large scale production of electric power. In such systems the total quantity of cadmium required becomes a problem, since cadmium is in short supply in the United States and is expensive. Reduction of the feasible thickness of cadmium compound required to fabricate a given area of cell is therefore crucial economically, and a reduction of thickness of CdS layer by an order of magnitude or more renders economically feasible a large scale power generator of the photo-voltaic type, which otherwise is not economically feasible. Utilization of minimum cadmium per unit area of cell is rendered feasible by utilization of a CuSO.sub.4 --Cu combination in the cell.
It is, accordingly, a primary object of the present invention to provide a photovoltaic cell which utilizes minimum weight of cadmium per unit area and which can therefore be economically utilized as a power source in a large scale electrical power generation system. This same objective is subserved by providing a cell which has only coplanar electrodes, and also in terms of time required to fabricate a given area of cell, a twenty micron layer requiring twenty times as much spray time as does a one micron layer, in forming the requisite cadmium sulfide microcrystalline layer on a substrate.
In the U.S. Patent to Hill et al. U.S. Pat. No. 3,148,084, issued Sept. 8, 1964, a method is taught for forming a layer of cadmium sulphide microcrystals on a glass substrate. Essentially, the method involves spraying the glass substrate, while the layer is hot, with a cadmium salt-thiourea complex, i.e., cadmium chloride plus a thiourea, in suitable proportions. The teaching of the patent is that the glass may be heated by means of a hot plate, and that the spraying may take place in the atmosphere. We have found that precisely uniform temperature of the glass plate is essential and that a hot plate is not able to heat a glass plate uniformly because the hot plate and the glass plate do not make prefect contact throughout, and that even slight nonuniformities of temperature of the glass substrate during spraying produce anomalous areas of the layer of CdS, which can render an entire photovoltaic cell inoperative.
The layer of CdS must grow in the form of many tiny crystals, the axes of which are predominantly parallel. Application of the sprayed materials at a uniform, and sufficiently slow rate, is important, as is uniformity of temperature, to assure uniformity of crystal growth rate and of orientation over the entire glass plate.
In order to provide uniformity of temperature over the entire glass plate, according to one feature of the invention, the plate is floated during coating in melted tin, at over 700.degree. F.
In accordance with the teaching of U.S. patent to A. E. Carlson, U.S. Pat. 2,820,841, issued Jan. 21, 1958, it is necessary to superimpose Cu.sub.2 S to form a heterojunction on a layer of CdS micro-crystals formed on Nesa glass. This may be accomplished according to the present invention by spraying on the layer of CdS while the latter is at about 200.degree. F. to 300.degree. F., a small quantity of copper chloride and of thiourea, which, in impinging against the hot CdS, forms a layer of Cu.sub.2 S about 1000 A.degree. thick thereover. Since the layer of Cu.sub.2 S is formed by spraying cold materials which form only on contact with the CdS layer, a flat layer is formed which so combines with the exposed parts of the CdS crystals as to form the required photovoltaic junction.
At this point, according to the teaching of Carlson, supra, it would appear only necessary to apply an electrode to the Cu.sub.2 S layer, and a lead to the Nesa glass, to complete the fabrication of a photovoltaic cell. A cell so fabricated is not satisfactory. Nesa glass is conductive only because it has a coating of tin oxide. But, tin oxide has high resistance taken along the surface of the glass, so that a great deal of the energy generated by the cell is lost in the tin oxide layer, and this is the more true the larger is the cell. The problem can be ameliorated by breaking up larger cells into smaller cells, as in FIG. 4 of Carlson et al, but only at the cost of added complexity of fabrication.
According to the present invention, we deposit over the layer of Cu.sub.2 S a layer of CuSO.sub.4, by spraying, and over the latter deposit two separated electrodes of copper and zinc, respectively. On heating the cell to about 500.degree. F. for about 12 minutes, the CuSO.sub.4 gives up oxygen to the copper electrode, forming a CuO rectifying like junction, which is conductive for current flow out of the copper electrode, but the zinc diffuses down through the layers which it overlies, sometimes down to the layer of tin oxide and sometimes only to but not through the CdS layer. In any event, it has been found that if the tin oxide layer be considered to be at ground potential, the copper electrode may be at 420 mv., while the zinc electrode may be, in some samples, at 0 mv., and in others at minus 20 mv. The copper and zinc electrodes may be interdigitated and the interdigitations located sufficiently close together that the return paths for current internally of the cell along the tin oxide layer can be short, yet the electrode and lead system can remain simple, and easy to fabricate, requiring no etching through the CdS -- Cu.sub.2 S sandwich.
The CuSO.sub.4 -- Cu junction serves to prevent flow of reverse currents through holes which sometimes develop in the CdS layer. Such holes may occur due to defects of the fabricating process, and when they occur the cell is defective because a short circuit path to the SnO.sub.x is then available, and it is the presence of this junction which renders feasible the reduction of cadmium usage by an order of magnitude, in comparison with prior art cells.